1. Field of the Invention
The present invention relates to a protective tube for a thermocouple for directly measuring the temperature of a molten metal, and a method of producing the same.
2. Description of the Embodiment
Widely used for protecting a thermocouple for directly measuring the temperature of a molten metal such as aluminum have conventionally been protective tubes consisting of cast iron tubes coated with corrosion-resistant ceramic powders. Such protective tubes, however, are prone to have their metal materials dissolved in the molten metal being measured, thus degrading the molten metal. Also, since the corrosion-resistant ceramic coatings do not have sufficient adhesion to the protective metal tubes, they should be applied every day, thus making the temperature measurement operations costly. A further disadvantage is that since they are made of metal such as cast iron, they are relatively heavy and so not easy to handle. On the other hand, as for a cast iron melt, a thermocouple is directly dipped in the melt without using a protective tube to measure its temperature quickly. In this case, however, the thermocouple is somewhat dissolved in the melt, making it impossible to conduct temperature measurement continuously for a long period of time. As an alternative method, there is a method of temperature measurement by utilizing a radiation thermometer. This method, however, fails to provide accurate temperature measurement.
Recently, protective tubes made of ceramics such as silicon nitride and silicon carbide have been replacing the metal protective tubes. These ceramic protective tubes are conventionally produced mainly by a reaction sintering method in which metal silicon is nitrided or carbonized. Therefore, they have a bending strength of no more than 30 kg/mm.sup.2 or so, insufficient for withstanding mechanical stress and shock during the temperature measurement operations. In addition, these ceramic protective tubes have relative low densities so that they have rough surfaces to which a molten metal is likely to adhere. Further, even those having a relatively good bending strength do not have high resistance to heat shock so that they are vulnerable to breakage due to heat shock.